This invention relates generally to electrochemical cells, and more specifically, to membrane electrode assemblies particularly useful in energy producing fuel cells and energy consuming electrosynthesis cells and methods of manufacture.
Electrochemical cells for converting chemical energy directly to electrical energy rely on chemical reactions between an electrolyte and a fuel. One well known fuel cell, namely the hydrogen/oxygen type fuel cell, relies on anodic and cathodic reactions which lead to the generation and flow of electrons and electrical energy as a useful power source for many applications. The anodic and cathodic reactions in a hydrogen/oxygen fuel cell may be represented as follows:
H2xe2x86x922H++2exe2x88x92 (Anode)
xc2xdO2+2exe2x88x92xe2x86x92H2O (Cathode)
Platinum catalysts are used to bring about both of the foregoing anodic and cathodic reactions. These catalysts typically in combination with activated carbon, organic binder and fluorocarbon polymers, such as Teflon(copyright) are bonded to either side of a proton conducting ion-exchange membrane to fabricate a membrane electrode assembly (MEA).
In the case of hydrogen/oxygen fuel cells, some improvements in catalyst application methods have been directed towards reducing the amount of costly platinum catalyst in formulations. Development of compositions, for example, was achieved by combining solubilized perfluorosulfonate ionomer (Nafion(copyright)), support catalyst (Pt on carbon), glycerol and water. This led to the use of low platinum loading electrodes. The following publications teach some of these methods for hydrogen/oxygen fuel cells: U.S. Pat. No. 5,234,777 to Wilson; M. S. Wilson, et al, J. App. Electrochem., 22 (1992) 1-7; C. Zawodzinski, et al, Electrochem. Soc. Proc., Vol. 95-23 (1995) 57-65; A. K. Shukla, et al, J. App. Electrochem., 19(1989) 383-386; U.S. Pat. No. 5,702,755 to Messell; U.S. Pat. No. 5,859,416 to Mussell; U.S. Pat. No. 5,501,915 to Hards, et al.
Fuel cells utilizing hydrogen as fuel, however, are not viewed as entirely suitable for portable applications, such as motorized vehicles, due mainly to problems associated with hydrogen storage. A more suitable alternative fuel would be a liquid fuel, such as a simple alcohol, like methanol, which can be used in a low cost dilute aqueous solution. There are reports in the literature of MEAs specifically for methanol fed fuel cells. The reactions at the electrodes are as follows:
CH3OH+H2Oxe2x86x92CO2+6H++6exe2x88x92 (Anode)
1xc2xdO2+6H++6exe2x88x92xe2x86x923H2O (Cathode)
While direct fed methanol fuel cells offer a good alternative to hydrogen type fuel cells, there are still technical problems associated with such cells. One key problem is proton membrane inefficiency allowing the permeation or transport of unreacted methanol fuel to the cathode side of the MEA where it undergoes oxidation from oxygen and catalyst to form carbon dioxide and water. This leads to fuel loss, as well as significant loss in fuel cell efficiency, i.e., voltage drop. A number of reports and patents in the literature describe methods for remedying methanol permeation across the membrane while retaining proton conductivity, as high as possible. Representative patents and other publications include: U.S. Pat. No. 5,523,177 to Kosek, et al; U.S. Pat. No. 5,672,439 and U.S. Pat. No. 5,874,182 to Wilkinson, et al; U.S. Pat. No. 5,945,231 to Narayanan, et al; U.S. Pat. No. 5,672,438 to Banerjee, et al; U.S. Pat. No. 5,992,008 to Kindler; U.S. Pat. No. 5,958,616 to Salinas, et al; U.S. Pat. No. 5,919,583 to Grot, et al; Cong Pu, et al, J. Electrochem. Soc., (142)7, (1995), L119-120;
While some progress has been made in the development of better performing solid polymer electrolytes for direct feed methanol fuel cells employing proton transporting membranes with lower methanol transport coefficient properties, most of the improved membranes still have been found to allow permeation of unreacted fuel to the cathode at sufficient levels to cause unacceptable voltage drops and wasted fuel.
Accordingly, there is need for improved membrane electrode assemblies for fuel cells and other electrochemical applications which allow for efficient transport of needed protons generated at the anode, while eliminating all or virtually all crossover of unreacted alcohol fuels from the anode to the cathode.
It is therefore one principal object of this invention to provide improved membrane electrode assemblies (MEA) with more efficient proton transport from the anode to the cathode while restricting the crossover of all, or virtually all unreacted fuel from the anode.
The MEAs of this invention are characterized by the following structural features:
An asymmetric membrane composite, and a cathode and anode in electrical contact with the composite forming a solid polymer electrolyte. In the case of methanol type fuel cell, anodes preferably include a deposited catalyst for converting the alcoholic fuel to needed protons, plus water and carbon dioxide. The asymmetric membrane is a composite structure comprising a non-porous, water and proton permeable, thin polymeric film, and a thicker porous support layer (stratum) or backing in juxtaposition therewith, wherein the alcohol oxidation catalyst is dispersed. The outer polymeric film layer and the porous support layer are preferably part of the same structure and exist as an integral film, wherein the outer film layer is a continuous film which retains the cation exchange properties, including proton transport characteristics and non-porous properties of the original unmodified polymer. Whereas the porous support layer backing is physically modified into a high surface area porous stratum having tortuous paths with most of the alcohol oxidation catalyst embedded therein for greater surface exposure to unreacted liquid fuel.
Deposition of the alcohol oxidation catalyst mainly in the porous support region provides important benefits, namely more effective proton transport due to continuous presence of cationic proton conducting polymer throughout the catalyst region; the presence of catalyst at the interface between the thin film separating membrane and the adjacent thicker porous support layer allowing for more complete oxidation of any unreacted alcohol; in the case of ion-exchange polymers possessing very low or near zero methanol diffusion coefficients, all or virtually all unreacted methanol is prevented from reaching the cathode, since the polymer is present throughout the catalyst region. The efficiency of the direct fed methanol fuel cell is thereby greatly enhanced.
The asymmetric composites are formed from cation-exchange polymers, preferably perfluorosulfonic acid types, such as DuPont""s Nafion brand of permselective cation-exchange membrane, or other similar performing water and proton transporting cationic type exchange materials. The asymmetric composites of the invention comprise the aforementioned non-porous, but water and proton transportable films preferably as a continuous, very thin outer layer allowing for the transport of protons formed at the anode, plus water to selectively crossover to the cathode where the protons react with oxygen. Similarly, this continuous film layer also serves to restrict the transport of residual amounts of remaining unreacted alcoholic fuel from crossing over to the cathode. That is, the permselective properties of the continuous, non-porous thin film portion of the asymmetric composite serve as a fail-safe in restricting the transport of still any unreacted fugitive alcoholic fuel, preventing it from crossing over and reacting at the cathode, and causing a reduction cell voltage.
Dimensionally, the non-porous film layer of the asymmetric composite is very thin relative to the porous support layer. Thicknesses of the non-porous film layer can vary generally from about 2 to about 10 xcexcm. The thicker porous support layer backing for the thin film also performs as a high surface area substrate or bed for the oxidation catalyst for greater surface contact with unreacted alcoholic fuel and for more efficient decomposition of all, or virtually all remaining unreacted alcohol in the alcohol-water fuel mixture. The thicker porous support layer with oxidation catalyst deposited therein facilitates the conversion of unreacted fuel to carbon dioxide, water and protons. Known precious metal catalysts, such as platinum/ruthenium metal, combinations of their oxides and alloys of platinum/ruthenium, including partially reduced platinum/ruthenium are deposited principally in the interior pores of the high surface area porous support layer for rapid conversion of residual unreacted alcoholic fuels. Through this process, undesirable crossover of unreacted fuel from the anode side to the cathode side is prevented or minimized and voltage fall-offs are prevented, or at least significantly reduced. Hence, the asymmetric catalytic membrane composites of the present invention provide for more efficient operating solid polymer electrolytes for use in the operation of direct fed methanol fuel cells, and other types of electrolytic cells.
Optionally, the asymmetric membrane composites of the present invention may also be used in-combination with supplemental ion-exchange membranes. For example, in the event of surface imperfections in the thin, non-porous polymeric film surface of the asymmetric composite. Such surface imperfections may allow transport of small, but performance impeding amounts of unreacted fuel from the anode to the cathode side of the MEA. This can be remedied by means of the supplemental cation exchange membrane layer applied to the thin outer film layer. The supplemental membrane restricts the transport of residual amounts of unreacted fuels to the cathode while still allowing the passage protons. The supplemental membrane may also be employed as a spacer device in electrochemical cells, when necessary. While useful, this supplemental membrane structure may result in somewhat higher internal resistances (IR) causing some voltage penalty.
It is still a further object of this invention to provide methods for manufacturing the improved membrane electrode assemblies as disclosed herein. One embodiment is a type of phase inversion process performed by the steps which comprise:
(i) forming a solution, preferably one which is concentrated comprising a cationic (, i.e., proton) transporting polymer by dissolution of a sufficient amount of the polymer in a first organic solvent to form the solution;
(ii) forming a catalyst-cationic polymer dispersion by mixing an appropriate catalyst with the solution of cationic polymer of step (i);
(iii) forming or casting a film from the catalyst-cationic polymer dispersion;
(iv) contacting the film with a second solvent which is miscible in the first organic solvent. However, the second solvent should be a non-solvent, e.g., water or alcohol, for the film. That is, the polymeric film should be insoluble in the second solvent, while the first solvent used in the solubilization of the polymer must be completely miscible with the non-solvent, and
(v) with the aid of the two solvent system the cast film is converted into a membrane with a very thin continuous outer skin layer, wherein the native non-porous, permselective proton transport properties are retained.
The two solvent system also forms the substantially thicker, porous support layer backing. In contrast to the very thin outer skin layer, the backing of the film is converted to a high surface area support layer having many tortuous paths. Accordingly, the porous support layer also performs as a high surface area substrate or bed for most of the oxidation catalyst introduced into the solution according to step (ii) for facilitating the conversion of unreacted alcohol to protons and water.
Pores in the thicker support layer are formed during slow evaporation of the solvent from the film at temperatures ranging from about 10 to about 50xc2x0 C. causing the polymer to solidify, i.e., undergo gelation.
As an alternative embodiment the catalyst-containing asymmetric membrane composites of this invention may also be prepared using salt solutions of precious metals, like platinum and ruthenium and converting the salts in-situ to their corresponding reactive metals. This alternative embodiment includes the steps of:
(i) forming a solution, more preferably a concentrated solution, comprising a cationic polymer by dissolution of a sufficient amount of the polymer in a first organic solvent;
(ii) casting a film from the solution comprising the cationic polymer;
(iii) contacting the film with a second solvent which is miscible in the first solvent. The second solvent should also be a non-solvent, e.g., water or alcohol, for the film, i.e., the film should be insoluble in the second solvent;
(iv) converting the film of step (iii) to an asymmetric membrane composite structure by solvent evaporation during gelation of the polymer. The film is converted to an asymmetric membrane. composite comprising the non-porous, preferably continuous, water and proton permeable very thin polymeric film and an adjacent thicker porous support layer therefor, free of catalyst;
(v) contacting the asymmetric membrane composite structure with a solution comprising metal salts for exchange by the membrane. The metals of the metal salts are any of those known to be suitable for catalyzing the oxidation of unreacted alcohol at least to protons, for example, and
(vi) converting the metal salts in-situ in the membrane to alcohol oxidizing catalyst. This useful alternative conducts the conversion reaction where most of the alcohol oxidizing catalyst is needed, namely in the high surface area porous support layer.
The conversion step of the process can be conveniently performed by treating the membrane with reducing agents, like sodium borohydride, stannous chloride, hydrazine, formic acid, and so on. In so doing, the exchanged/deposited salts are readily converted to reactive free metals, in-situ.
The improved MEAs of this invention not only find uses in direct fed methanol fuel cells and in hydrogen/air fuel cells, but are also suitable for use in other electrochemical cells and processes. For instance, in catalytic membrane reactors for conducting reactions. They can be used in hydrogenation processes, where one reactant is a gas which preferentially diffuses across the asymmetric membrane in the reactor. Examples of such reactions include the catalytic combining of hydrogen and oxygen to form hydrogen peroxide. The asymmetric membranes of the invention can also be used in separating highly explosive mixtures of hydrogen and oxygen.
Other representative applications for the MEAs of this invention include sensors, such as for the detection of alcohol, where a catalyst suitable for bringing about an electrochemical oxidation/reduction can be used when measured against a reference electrode. This provides a suitable means for measuring alcohol concentration.
The novel MEAs of this invention may also be employed in electrochemical syntheses reactions in electrolytic cells. They include hydrogen peroxide synthesis from oxygen reduction, or with redox catalysts, electrochemical hydrogenation of oil, and many other electrochemical syntheses requiring catalytic membranes which act as ionic conductors, as well as separators between anodes and cathodes.